I think Carmax checks compression. Anyway, it's a little early to bail. Low compression has 4 possibilities:
1. Worn, broken or stuck rings
2. Stuck open valves
3. Leaking headgasket
4. Cracked block

(Or loose 2nd spark plug in the case of 2-plug engines like yours)

Only number 4 requires a new engine, and it's extremely unlikely. Like Arthur said, the shop should have told you the next steps they need to take to isolate the cause.
It won't be cheap, but way less than a new engine.
BTW, only Chevron, Texaco and Shell use additives to combat top end gunk buildup. The others just specialize in excessive profit, whatever your cost.

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